碳酸钙同质多象转变过程中氧同位素分馏的实验研究

本文通过“一步法”和“两步法”的“附晶生长法”分别合成了碳酸钙矿物，对其进行了同质多象转变过程中的氧同位素分馏行为的研究。同时通过真空条件下的同质多象转变实验，对文石和方解石的酸分馏系数的差异进行了实验验证。结果发现：①在50℃和70℃下，如果文石与水之间的氧同位素分馏未达到平衡，由母体文石经过同质多象转变生成的次生方解石完全继承的母体矿物的氧同位素组成。如果文石与水之间的氧同位素分馏达到平衡，由互过同质多象转变生成的次生方解石部分继承母体文石的氧同位素组成，并且次生方解石相对于母体文石富集^18O。②在0℃和25℃下，“一步法”的“附晶生长法”的实验结果表明，由六方方解石经过同质多象转变生成的次生方解石也完全继承母体矿物的氧同位素组成，并且在该温度下，矿物与水之间的氧同位素分馏与温度无关。     

碳酸钙-水体系氧同位素平衡及稳态分馏的低温实验研究

采用“一步”和“两步”的直接沉淀法和附晶生长法在 5 0℃和 70℃分别合成碳钡矿和文石 ,测定不同条件下合成矿物与水之间的氧同位素分馏 ,结果显示 ,文石—水体系氧同位素分馏机理分两步 :(1) [CO3 ]2 - 与H2 O的氧同位素交换和平衡 ,此过程是文石 水氧同位素平衡的决速率步骤 ;(2 )与H2 O平衡以后的 [CO3 ]2 - 与Ca2 +]结合生成文石 ,此过程体现矿物形成过程中氧同位素分馏的结构效应。在此基础上 ,采用缓慢沉淀法和“两步”的附晶生长法获得了 0～ 70℃的文石 水体系氧同位素平衡分馏方程。采用“一步”和“两步”的附晶生长法在 5 0℃和 70℃合成文石 ,文石在溶液中经同质多象转变成次生方解石 ;结合文献数据 ,获得 0～ 70℃范围内的方解石 水体系稳态氧同位素分馏方程。     

白云鄂博碳酸岩墙碳氧同位素地球化学

对内蒙古白云鄂博 REE- Fe- Nb矿床周围碳酸岩墙中共存的方解石和白云石进行了 C和 O同位素分析。结果表明，方解石和白云石的δ 13C值变化范围一致，均为－ 3.5‰～－ 7.3‰，落在正常地幔δ 13C值范围 (－ 5‰± 2‰ )内；而它们的δ 18O值可分为两组，第Ⅰ 组为 9.5‰～ 18.0‰，第Ⅱ 组为 20.6‰～ 22.6‰，均远大于正常地幔δ 18O值范围 (5.7‰± 1.0‰ )。第Ⅰ 组低δ 18O值样品中共存白云石与方解石之间的 C和 O同位素分馏均为负值，因此处于热力学不平衡状态，指示它们自形成后受到过后期热液蚀变，与先前的岩石学观察一致。相反，第Ⅱ 组高δ 18O值样品中白云石与方解石之间的 C和 O同位素分馏均为正值，处于热力学平衡状态，指示它们自形成后未受到后期热液蚀变，因此可能沉淀于晚期低温高δ 18O值流体。第Ⅰ 组碳酸岩墙中白云石的 C和 O同位素组成不呈线性分布，指示碳酸岩浆并非由幔源碳酸盐与沉积碳酸盐混合形成。应用水-岩交换模型计算得到，第Ⅰ 组碳酸岩在侵位后经历了碳酸岩浆期后热液的不均一蚀变，蚀变温度约在 220～ 800℃之间，蚀变流体的 CO2/H2O比值较小 (1/500)，但水 /岩比值变化较大 (10～ 400)。由于低温下方解石与热液之间的碳氧同位素交换速率大于白云石，导致这部分碳酸     

湖泊沉积碳酸盐氧同位素影响因素的定量评估

湖泊自生碳酸盐δ18O是重建湖泊演化历史的重要代用指标,然而由于湖泊沉积物质来源的多元性和沉积过程的复杂性,所获沉积物碳酸盐的δ18O往往具有多解性.在嘎顺诺尔古湖相沉积小狐山剖面,其剖面590 ～500cm层段湖泊沉积物碳酸盐矿物中的方解石逐渐被白云石取代,同时碳酸盐δ18O呈现异常偏重趋势,其幅度达6.1‰.本文以此为例,通过对沉积物碳酸盐矿物类型和碳酸盐δ18O的分析,结合沉积物粒度、TOC和有机碳同位素等代用指标,对碳酸盐δ18O的影响因素进行了定量评估.对小狐山剖面而言,结果显示湖水蒸发浓缩效应和碳酸盐矿物相变均会导致碳酸盐δ18O发生变化,其中碳酸盐矿物相变引起的分馏作用是影响δ18O发生非环境效应的最主要因素之一.小狐山剖面沉积物中白云石较方解石富集δ18O约7.65‰～ 11.02‰,由此估算白云石与方解石之间的分馏作用可以解释δ18O波动约70％.本文研究结果揭示了自然条件下生成的不同碳酸盐矿物相变存在氧同位素分馏,在进行古环境重建的过程中必须对此进行重视和评估.     

白云鄂博富稀土元素碳酸岩墙的 碳和氧同位素特征

重点解剖了一条距白云鄂博超大型REE-Nb-Fe矿床东矿北东方向2 k m、切割白云鄂 博群H1及H3岩性段的细粒方解石碳酸岩岩墙的碳和氧同位素地球化学特征。结果表明,碳酸 岩的碳同位素组成变化范围较小(δ13C值为－6.6‰ ～ －4.6‰),与正常地幔碳δ 13C值－5±2‰一致;而氧同位素组成变化范围较大(δ18O值为11.9‰～17.7‰ ),显著高于地幔的δ18O值5.7±1.0‰,表明碳酸岩浆在结晶过程中或之后曾与 低 温热液流体发生了同位素交换。碳酸岩墙中白云石与方解石之间的碳和氧同位素分馏均小于 0‰,处于不平衡状态,说明该碳酸岩墙中的白云石与方解石并非同成因矿物,白云石可能 为次生成因的。     

南海台西南区碳酸盐岩矿物学和稳定同位素组成特征——天然气水合物存在的主要证据之一

南海台西南区是中国南海中天然气水合物赋存的最有利场所。研究表明,该区的碳酸盐岩主要以结壳、烟囱的形式出现,结壳的裂隙或孔洞中常常充填有淡黄-白色的文石晶体。碳酸盐岩中自生碳酸盐矿物主要为文石、高镁方解石,少量白云石、铁白云石和菱铁矿。扫描电子显微镜(SEM)分析表明,文石主要呈针状、长柱状、放射束状,高镁方解石呈颗粒状。碳酸盐岩的碳同位素δ13C值主要在-56·878‰~-32·829‰PDB之间,大多数小于-40‰PDB,显示了生物甲烷成因碳源的特征;氧同位素δ18O值在2·1875‰~5·045‰PDB之间,主要在4‰PDB以上,这种较重的氧同位素比值表明,天然气水合物分解产生的富18O水体可能是碳酸盐岩沉淀的流体源。矿物学和碳氧稳定同位素研究表明,南海台西南区的碳酸盐岩为细菌性甲烷成因碳酸盐岩,可能与天然气水合物有关,显示了该区水合物存在的可能性很大。     

文石-水体系氧同位素分馏系数的低温实验研究

采用缓慢分解法和“两步法”的附晶生长法,在低温(0℃～70℃)下实验合成纯文石型碳酸 钙矿物,以XRD和SEM技术对合成矿物的相组成和形貌进行了鉴定。将XRD与SEM及氧同位素分 析技术相结合,研究了文石的生成速率与氧同位素分馏之间关系。对0℃、25℃和50℃条件 下采用缓慢分解法合成的文石进行SEM观察发现,随着温度升高,矿物生成速率加快,氧同 位素分馏逐渐趋于不平衡,导致50℃条件下获得的文石-水体系氧同位素分馏是一种不平衡 分馏,而0℃和25℃条件下获得的低值代表平衡分馏。将0℃和25℃以下采用缓慢分解法获得 的文石-水体系分馏低值与采用“两步法”的附晶生长法在50℃和70℃条件下获得的文石- 水体系平衡分馏数据相结合,得到0℃～70℃范围内文石-水体系氧同位素平衡分馏方程为 ：103lnα=20.41×103T－41.42。这个实验结果不仅与增量方法理论计算结 果一致,而且与前人低温实验获得的文石或文石与方解石混合相碳酸钙-水体系,以及生物 成因文石-水体系的氧同位素分馏结果相近。这是首次根据实验确定的无机成因文石-水体 系热力学平衡氧同位素分馏系数,因此对于无机成因文石在古沉积环境和古气候研究中的应 用具有重要参考价值。     

不同碳酸盐与磷酸反应的C、O同位素效应

不同阳离子碳酸盐与磷酸反应放出的CO2之间的氧分馏系数在20-90℃下与1/T2呈线性函数关系，而C同位素分馏则与温度无关。C、O同位素分馏程度与碳酸盐中的阳离子有关。利用不同阳离子碳酸盐与磷酸反应的速度不同，可以直接提取混合碳酸盐样品的CO2。     

矿物O同位素地球化学理论

应用增量方法不仅能够从理论上准确计算所有固体矿物的O同位素分馏系数，而且能够定量预测；（1）热力学平衡条件下共生矿物之间的18O富集顺序；（2）岩石化学成分与O同位素组成之间的关系；（3）矿物结构变化对O同位素组成的影响；（4）同质多相转变和矿物反应中的氧同位素继承性。本文对这些规律性预测进行了概要性介绍，并给出实例予以说明。     

碳酸钙-水体系氧同位素分馏系数的低温实验研究

碳酸钙是古气候和沉积岩稳定同位素地球化学研究中最常用的矿物 ,因此对碳酸钙水体系氧同位素分馏系数的实验校准已成为稳定同位素地球化学诞生以来的热点和前沿课题。但由于碳酸钙在自然界存在 3种同质多象变体 (方解石、文石和六方方解石 ) ,使人们对碳酸钙矿物与水之间氧同位素分馏系数的实验测定结果存在较大差别 ,当应用到同位素地质测温时 ,会给出显著不同的温度值。正确选用合理的方解石水或文石水体系分馏曲线 ,对低温和环境地球化学研究和应用具有重要价值。文章系统总结和评述了碳酸钙水体系氧同位素分馏系数实验校准的历史、方法和结果 ,对前人在表达方式上的不一致进行了统一 ,对氧同位素分馏的盐效应、动力氧同位素分馏效应和同质多象转变过程中的氧同位素继承性进行了讨论。通过对前人大量实验数据的系统处理并与理论计算相比较 ,推荐了热力学上平衡的方解石水体系氧同位素分馏方程 ,而对于文石水体系 ,理论计算结果尚有待于实验证实。     

On the Direction and Magnitude of Oxygen Isotope Fractionation Between Calcite and Aragonite at Thermodynamic Equilibrium

Oxygen isotope fractionation factors between calcium carbonates and water have been applied to ancient marine geochemistry principally for the purpose of geothermometry. The problem was encountered, however, with respect to the direction and magnitude of oxygen isotope fractionation between calcite and aragonite at thermodynamic equilibrium. This basically involves sound understanding of both thermodynamics and kinetics of oxygen isotope fractionation between inorganically precipitated carbonate and water at low temperatures. Thus the crucial issues are to acknowledge the processes of chemical reaction and isotopic exchange during precipitation of CaCO₃ minerals in solution, the kinetic mechanism of isotope equilibrium or disequilibrium, the effect of polymorphic transition from metastable aragonite to stable calcite under hydrous or anhydrous conditions, and the presence or absence of isotope salt effect on oxygen isotope exchange between carbonate and water in response to the hydrous or anhydrous conditions at thermodynamic equilibrium. Because good agreements exist in carbonate–water oxygen isotope fractionation factors between theoretical calculations and experimental determinations, it is encouraging to applying the thermodynamic and kinetic data to isotopic paleothermometry and geochemical tracing.     

Response to the Comment by J. Horita and R.N. Clayton on “The studies of oxygen isotope fractionation between calcium carbonates and water at low temperatures”

The apparent inconsistency in calcite-water fractionation does occur between the arithmetic combination of Zhou and Zheng [Zhou G.-T., and Zheng Y.-F. (2003) An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures. Geochim. Cosmochim. Acta67, 387-399] and the experimental determination of Zhou and Zheng [Zhou G.-T., and Zheng Y.-F. (2005) Effect of polymorphic transition on oxygen isotope fractionation between aragonite, calcite and water: a low-temperature experimental study. Am. Mineral90, 1121-1130]. To resolve this issue is to acknowledge whether or not the isotope salt effect of dissolved minerals would occur on oxygen isotope exchange between water and the minerals of interest. The question is whether or not a term of mineral-water interaction should be taken into account when calculating mineral-water 10³ln α factors by an arithmetic combination between theoretical 10³ln β factors for mineral and water, respectively. The hydrothermal experiments of Hu and Clayton [Hu G.-X., and Clayton R.N. (2003) Oxygen isotope salt effects at high pressure and high temperature, and the calibration of oxygen isotope geothermometers. Geochim. Cosmochim. Acta67, 3227-3246] demonstrate the absence of isotope salt effect on the oxygen isotope fractionation between calcite and water, and this abnormal behavior reasonably explains the so-called inconsistency in the calcite-water fractionations of Zhou and Zheng (2003, 2005). We argue that the mineral-water correction is still necessary for calculation of fractionations in mineral-water systems. New experimental data for oxygen isotope fractionations involving dolomite and cerussite are consistent with the calculations of Zheng [Zheng Y.-F. (1999a) Oxygen isotope fractionation in carbonate and sulfate minerals. Geochem. J.33, 109-126], but also shed light on the assumptions used in modifying the increment method. We argue that the modified increment method has developed into a theoretical mean of predictive power for calculation of oxygen isotope fractionation factors for crystalline minerals of geochemical interest.     

Theoretical calculation of oxygen isotope fractionation factors in carbonate systems

Using established methods of statistical mechanical calculation and a recent compilation of vibrational frequency data, we have computed oxygen isotope reduced partition function ratios (β values) for a large number of carbonate minerals. The oxygen isotope β values of carbonates are inversely correlated to both the mass and radius of the cation bonded to the carbonate anion but neither correlation is good enough to be used as a precise and accurate predictor of β values. There is an approximately 0.6% relative increase in the β values of aragonite per 10 kbar increase in pressure. These estimates of the pressure effect on β values are broadly similar to those deduced previously for calcite using the methods of mineral physics. In comparing the β values of our study with those derived recently from first-principles lattice dynamics calculations, we find near-perfect agreement for calcite and witherite (<0.3% deviation), reasonable agreement for dolomite (<0.9% deviation) and somewhat poorer agreement for aragonite and magnesite (1.5-2% deviation). In the system for which we have the most robust constraints, CO₂-calcite, there is excellent agreement between our calculations and experimental data over a broad range of temperatures (0-900 °C). Similarly, there is good to excellent correspondence between calculation and experiment for most other low to moderate atomic mass carbonate minerals (aragonite to strontianite). The agreement is not as good for high atomic mass carbonates (witherite, cerussite, otavite). In the case of witherite and cerussite, the discrepancy may be due, in part, to our calculation methodology, which does not account for the effect of cation mass on the magnitude of vibrational frequency shifts associated with heavy isotope substitution. However, the calculations also reveal an incompatibility between the high- and low-temperature experimental datasets for witherite and cerussite. Specifically, the shapes of fractionation factor versus 1/T² curves in the calcite-witherite and calcite-cerussite systems do not conform to the robust constraints on the basic shape of these curves provided by theory. This suggests that either the high- or low-temperature datasets for both minerals is in error. Dolomite-calcite fractionation factors derived from our calculations fall within the wide range of fractionations for this system given by previous experimental and natural sample studies. However, our compilation of available low-temperature (25-80 °C) experimental data reveal an unusual temperature dependence of fractionations in this system; namely, the data indicate an increase in the magnitude of fractionations between dolomite (or proto-dolomite) and calcite with increasing temperature. Such a trend is incompatible with theory, which stipulates that fractionations between carbonate minerals must decrease monotonically with increasing temperature. We propose that the anomalous temperature dependence seen in the low-temperature experimental data reflect changes in the crystallinity and degree of cation ordering of the dolomite phase over this temperature interval and the effect these changes have on the vibrational frequencies of dolomite. Similar effects may be present in natural systems at low-temperature and must be considered in applying experimental or theoretical fractionation data to these systems. In nearly all cases, carbonate mineral-calcite fractionation factors given by the present calculations are in as good or better agreement with experimental data than are fractionations derived from semi-empirical bond strength methods.     

Oxygen isotope salt effects at high pressure and high temperature and the calibration of oxygen isotope geothermometers

The influence of NaCl, CaCl₂, and dissolved minerals on the oxygen isotope fractionation in mineral-water systems at high pressure and high temperature was studied experimentally. The salt effects of NaCl (up to 37 molal) and 5-molal CaCl₂ on the oxygen isotope fractionation between quartz and water and between calcite and water were measured at 5 and 15 kbar at temperatures from 300 to 750°C. CaCl₂ has a larger influence than NaCl on the isotopic fractionation between quartz and water. Although NaCl systematically changes the isotopic fractionation between quartz and water, it has no influence on the isotopic fractionation between calcite and water. This difference in the apparent oxygen isotope salt effects of NaCl must relate to the use of different minerals as reference phases. The term oxygen isotope salt effect is expanded here to encompass the effects of dissolved minerals on the fractionations between minerals and aqueous fluids. The oxygen isotope salt effects of dissolved quartz, calcite, and phlogopite at 15 kbar and 750°C were measured in the three-phase systems quartz-calcite-water and phlogopite-calcite-water. Under these conditions, the oxygen isotope salt effects of the three dissolved minerals range from ∼0.7 to 2.1‰. In both three-phase hydrothermal systems, the equilibrium fractionation factors between the pairs of minerals are the same as those obtained by anhydrous direct exchange between each pair of minerals, proving that the use of carbonate as exchange medium provides correct isotopic fractionations for a mineral pair.When the oxygen isotope salt effects of two minerals are different, the use of water as an indirect exchange medium will give erroneous fractionations between the two minerals. The isotope salt effect of a dissolved mineral is also the main reason for the observation that the experimentally calibrated oxygen isotope fractionations between a mineral and water are systematically 1.5 to 2‰ more positive than the results of theoretical calculations. Dissolved minerals greatly affect the isotopic fractionation in mineral-water systems at high pressure and high temperature. If the presence of a solute changes the solubility of a mineral, the real oxygen isotope salt effect of the solute at high pressure and high temperature cannot be correctly derived by using the mineral as reference phase.     

Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate ‘clumped isotope’ analysis. The CO₂ recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H₂CO₃ reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of ¹³C-¹⁸O-containing isotopologues (‘clumped’ isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72‰, 0.220‰, 0.137‰, 0.593‰ for, respectively, ¹⁸O/¹⁶O ratios (1000 lnα^∗) and three indices that measure proportions of multiply-substituted isotopologues . We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of ¹³C-¹⁸O bonds (the measured change in Δ₄₇ = 0.23‰) during phosphoric acid digestion of calcite at 25 °C.We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H₂CO₃ interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of ¹³C-¹⁸O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of for different carbonate minerals. Combined with previous theoretical evaluations of ¹³C-¹⁸O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H₂CO₃.     

Kinetic mechanism of oxygen isotope disequilibrium in precipitated witherite and aragonite at low temperatures: an experimental study

To study what dictates oxygen isotope equilibrium fractionation between inorganic carbonate and water during carbonate precipitation from aqueous solutions, a direct precipitation approach was used to synthesize witherite, and an overgrowth technique was used to synthesize aragonite. The experiments were conducted at 50 and 70°C by one- and two-step approaches, respectively, with a difference in the time of oxygen isotope exchange between dissolved carbonate and water before carbonate precipitation. The two-step approach involved sufficient time to achieve oxygen isotope equilibrium between dissolved carbonate and water, whereas the one-step approach did not. The measured witherite-water fractionations are systematically lower than the aragonite-water fractionations regardless of exchange time between dissolved carbonate and water, pointing to cation effect on oxygen isotope partitioning between the barium and calcium carbonates when precipitating them from the solutions. The two-step approach experiments provide the equilibrium fractionations between the precipitated carbonates and water, whereas the one-step experiments do not. The present experiments show that approaching equilibrium oxygen isotope fractionation between precipitated carbonate and water proceeds via the following two processes:

1.

Oxygen isotope exchange between [CO₃]²⁻ and H₂O:

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Oxygen and carbon isotopic constraints on the development of eclogites, Holsnøy, Norway

Eclogite formation on the island of Holsnøy required the addition of water to anhydrous granulite-facies protoliths. In order to assess this process, oxygen and carbon isotope ratios of whole rock powders and mineral separates from eclogites and granulites have been measured. Whole rock oxygen isotope ratios range from 7.3 to 6.0%. SMOW in granulites (average = 6.38%.) and 7.2 to 6.1%. in eclogites (average = 6.55%.). Field relations permit identification of the granulite protolith of eclogites. Oxygen isotope measurements show shifts of up to 0.5%. between some eclogites compared to their corresponding granulite protoliths, indicating open system and locally heterogeneous fluid behavior. Mineral pair fractionations in the eclogites show disequilibrium, are incompatible with slow cooling and diffusive exchange between phases, and suggest that open system fluid movement continued after eclogite-facies metamorphism. Carbonate is also present in some of the eclogites as a primary mineral (dolomite) and as part of a retrograde assemblage (calcite). Textural evidence suggests that carbonate formation occurred during and after eclogite formation, however all measured carbonate is out of isotopic equilibrium with eclogite facies minerals, due to the influx of retrograde fluids. Massive calcite marble pods, containing amphibolite facies cale-silicate minerals, have average δ¹⁸O of 9.5 ± 0.6%., while calcite in retrograded eclogites has δ¹⁸O 17.7 ± 2.7%., The δ¹³C (≈ −4 ± 0.8%.) is indistinguishable between these two groups.

Both whole rock and carbonate stable isotope data are interpreted as indicating a continued history of fluid infiltration during and after peak eclogite facies metamorphism. The most probable source of fluids are from dewatered sediments tectonically juxtaposed during the Caledonian orogeny.